The present invention relates to a modified latex containing phosphorus surface groups. This modified latex is useful as a coating for substrates and as a binder in aqueous systems containing inorganic fillers commonly employed in paper coatings, carpet backings, wallboards and other substrates.
In general mineral coatings or fillers are applied to paper to cover the individual fibers of the paper and fill interstices between fibers, thus rendering the surfaces of the paper more level and uniform in texture. Mineral coatings or fillers are also employed in aqueous dispersions of fiber to form a composite sheet. This process is described in U.S. Pat. No. 4,225,383.
In order to suitably bind the mineral coatings or fillers to the substrate, latexes are employed. Latexes bind the filler such that it will not be removed by the pull of printing ink. Besides printability, the latexes also help to influence many other qualities of the paper such as appearance and strength.
The ability of the latex to function as a binder is very important to the final quality of the finished substrate, i.e., dry and wet pick in paper coatings. Generally in the art of paper coating, the latex component is admixed with a filler in an aqueous suspension and mechanically applied to the substrate to be coated. Alternatively, in the art of composite sheet formation, a latex is added to an aqueous dispersion of fiber and inorganic filler which is destabilized to form a fibrous agglomerate which is deposited onto a screen or other porous means to form a sheet therefrom.
For various reasons, it is desirable to maximize the retention level of inorganic fillers on the substrate to be coated. One way to accomplish this is to use a latex which interacts well with the filler. Improved interaction increases retention which has the benefits of reducing the level of flocculant required to destabilize the filler from solution and of reducing the stickiness of the filler and latex to equipment which occurs when high flocculant levels are used. This, in turn, reduces the maintenance needed in the manufacture of composite sheets. The latter benefit is also realized with respect to cleaning up excess residual filler left behind in the aqueous solution by the latex after deposition. Another advantage to increasing filler retention and thereby content is, of course, economical, as higher filler content usually translates to less expensive final products.
It is well known in the art, however, that while a particular amount of filler can enhance the properties of a coated product, higher amounts can cause loss of strength due to decreased structural integrity. It, therefore, would be desirable to develop a method which increased filler retention without reducing physical properties or ideally to increase the strength of substrates with higher filler levels. Accordingly, it would be desirable to develop a latex which had improved interaction with inorganic fillers to enable better retention of the fillers which, in turn, deposit themselves on a substrate to be coated, with little or no reduction in strength.